Bog is a pRb binding protein, whose over-expression in normal rat liver epithelial (RLE) cells conferred resistence to the growth inhibitory effect of TGF-b1 and ultimately leads to a transformed phenotype in vitro. We investigated the expression and cellular localization of BOG transcripts in two models of liver regeneration, partial hepatectomy (PH) alone and 2-acetylaminofluorene (AAF) /PH model. In both models BOG expression increases during liver regeneration. Prior to PH, BOG transcripts were observed in bile epithelial cells and sinusoid lining cells, but not detected in hepatocytes. After PH, expression of BOG is seen also in hepatocytes. In the AAF/PH model, characterized by selective proliferation of oval and stellate cells, increased levels of BOG transcripts are primarily observed in oval and stellate cells with low expression seen in the non-proliferating hepatocytes. The increased expression of BOG was coincidental with high level of TGF-b1 expression observed in the AAF/PH model. We propose that transient increase in BOG expression in vivo may facilitate expansion of the early progeny of hepatic stem cells. - Apoptosis, Cytokines, Growth factors, Hepatocarcinogenesis, Liver, Pancreas, Regeneration, Stem cells, TGF- beta-1,